In Trailblazing Tokyo Governor, Japan Meets Its Great Disrupter

As a young television newscaster fluent in Arabic, she interviewed the Libyan dictator Col. Muammar el-Qaddafi. When she asked sharp questions of a future prime minister, he recruited her to run for Parliament, and she won. As environment minister, she pushed traditionally formal Japanese businesses to let workers wear casual clothes in the summer to lower air-conditioning bills. During Prime Minister Shinzo Abe’s first stint in the post, she became the first woman to be named defense minister, and though felled by a scandal, she returned last year to become the first female governor of Tokyo.

Now Yuriko Koike is shaking things up again.

Ms. Koike founded a new national party two weeks ago, and Japan is watching to see whether she goes for broke and runs for parliamentary office herself in elections this month, which would put her in position to challenge Mr. Abe.

Ms. Koike, 65, who was elected governor of Tokyo just over a year ago, has made no secret of her ambition to become Japan’s first female prime minister. This month’s lower-house election, called early by Mr. Abe in an effort to consolidate his power, provides an opportunity for her.

So far, Ms. Koike has insisted that she will not run for national office. “I was given a passionate offer, but as I have repeatedly said, I don’t have any intention to run in the lower-house poll,” she told reporters as recently as Thursday.

But Ms. Koike has until Tuesday, the official start of the election campaign, to make a final declaration.

Whatever she decides, she has injected considerable drama into what originally looked like a rubber-stamp election for Mr. Abe. The prime minister had been hoping to capitalize on tensions over North Korea and the existing opposition’s political weakness to distract from scandals that have threatened to undermine him all summer.

Ms. Koike’s rise through the male-dominated world of Japanese politicscomes as other women have recently struggled. This summer, two prominent women — the leader of the opposition Democratic Party and Mr. Abe’s second female defense minister — resigned, raising questions about the persistence of the glass ceiling.

By founding a new party that could threaten Mr. Abe’s governing Liberal Democrats, Ms. Koike could upset a political order in which one party has dominated for most of Japan’s postwar era. But while some analysts have labeled Ms. Koike a populist, it is not clear whether she can tap into the kind of support that carried Donald J. Trump to the presidency in the United States.

Just hours before Mr. Abe called late last month for the early election, Ms. Koike unveiled her new party — Kibou no To, or Party of Hope — calling it a “reformist, conservative” alternative to “vested interests.”

Her announcement set off a cascade of political dominoes: the opposition Democratic Party first offered to free all of its candidates to run under Ms. Koike’s umbrella, but after she said she would submit them to a litmus test, the left-wing branch of the Democrats formed yet another new party.

“This is an unprecedented level of disruption in Japanese politics,” said Tomoaki Iwai, a professor of politics at Nihon University in Tokyo.

If Ms. Koike decides to run and her party wins a majority, she could end up as prime minister. But even if the Party of Hope does not become the governing party, it could cut into the Liberal Democrats’ commanding majority, which could dash Mr. Abe’s hopes of running for a third consecutive term as leader of the Liberal Democrats and becoming Japan’s longest-serving prime minister.

As governor, Ms. Koike has proved brilliant at keeping media attention trained on her, commanding respect in crisp, bright suits in a sea of black-suited men.

In Tokyo’s metropolitan elections in July, she founded an upstart local party that fielded 50 candidates, all but one of whom won seats in the local assembly. Mr. Abe’s Liberal Democrats wo...